What Buck said... In 60+ yrs of cattle production and 40+ yrs in veterinary medicine, I've only ever seen one live blackleg animal... and it was dead within 12 hrs. Every other one was found dead, either with no noted symptoms, or, in rare cases, the caretakers may have noticed it limping one day, and dead the next.
No doubt that it's a tragedy for you. But... the likelihood of gunshot, particularly in light of what you've shown us, is infinitesimally small.
As one of my professors once opined: "The most common things happen most often."
But, we'll never know...